THE XMM-NEWTON ABC GUIDE, STREAMLINED

OM (FAST Mode), GUI

Contents

Prepare the Data

Please note that the two tasks in this section (cifbuild and
odfingest) must be run in the ODF directory. These are the
only tasks with that requirement, and after this section, we will
work exclusively in our reprocessing directory.

Many SAS tasks require calibration information from the Calibration
Access Layer (CAL). Relevant files are accessed from the set of Current
Calibration File (CCF) data using a CCF Index File (CIF).

To make the ccf.cif file, first make sure the environment variables are set:

Next, call cifbuild from the SAS GUI. A window with the parameter
options will appear; the defaults should be fine, so just click "Run".

To use the updated CIF file in further processing, you will need to
reset the environment variable SAS_CCF:

setenv SAS_CCF /full/path/to/ODF/ccf.cif

The task odfingest extends the Observation Data File (ODF) summary
file with data extracted from the instrument housekeeping data files and
the calibration database. It is only necessary to run it once on any
dataset, and will cause problems if it is run a second time. If for some
reason odfingest must be rerun, you must first delete the earlier
file it produced. This file largely follows the
standard XMM naming convention, but has SUM.SAS
appended to it.
After running odfingest, you will need to reset the environment
variable SAS_ODF to its output file. To run odfingest and reset
environment variable, call odfingest from the SAS GUI. As before, a
pop-up window with the parameter options will appear; the defaults should
be fine, so just click "Run". (It is safe to ignore the warnings.)

To change the environmental variable, type

setenv SAS_ODF /full/path/to/ODF/full_name_of_*SUM.SAS

You will likely find it useful to alias these environment variable resets
in your login shell (.cshrc, .bashrc, etc.).

OM Artifacts and General Information

Before proceeding with the pipeline, it is appropriate to discuss the artifacts
that often affect OM images. These can affect the accuracy of a measurement by,
for example, increasing the background level. Some of these can be seen in Figure
1.

- Stray light. Background celestial light is reflected by
the OM detector housing onto the center on the OM field of view, producing a circular
area of high background. This can also produce looping structures and long streaks. - Modulo 8 noise. In the raw images, a modulo 8 pattern
arises from imperfections in the event centroiding algorithm in the OM electronics.
This is removed during image processing. - Smoke rings. Light from bright sources is reflected from the entrance
window back on the detector, producing faint rings located radially away from the center
of the field of view. - Out-of-time events. sources with count rates of several tens
of counts/sec show a strip of events along the readout direction, corresponding to
photons that arrived while the detector was being read out.

Users should also keep in mind some differences between OM data and X-ray data.
Unlike EPIC and RGS, there are no good time intervals (GTIs) in OM data; an
entire exposure is either kept or rejected. Also, OM exposures only provide direct
energy information when in grism mode, and the flat field response of the detector
is assumed to be unity.

Reprocess the Data

To reprocess the data in all exposures and filters, make a new working directory
and call omfchain from inside it.

cd ..
mkdir PROC
cd PROC

Then, close and re-open the SAS GUI, so that it will place the output files in the
new directory, and call omfchain. The default parameters are fine for most
data sets, so just click "Run".

There are two types of output files: those that start with F are
intermediate images or time series files; those that start with P
are products. The processed image in sky-coordinates from one exposure,
P0411081601OMS006SIMAGE1000.FIT, and the background-subtracted
light curve and various statistics associate with it,
F0411081601OMS006TIMESR1000.PS, are shown in Figure 1.

The background light curve in Figure 1 (right) is constant because omfchain
runs with the parameter bkgfromimage=yes by default, so that the background
light curve is found by using the imaging-mode data, instead of the fast-mode window.
This is preferable for even only moderately bright sources (count rate > 0.6 ct/s),
as the fast-mode window is small and any background measurement that uses it will likely
be contaminated with source photons. This is less of a concern if your source is faint,
in which case the background can by found from data in the fast-mode window by
setting bkgfromimage to no.

Verifying the Output

A good first step is to examine the light curve plot for both the source
and background, making sure they are reasonable: no isolated, unusually
high (or low) values, no frequent drop-outs, and a constant background
light curve if omfchain was run with bkgfromimage=yes. Users
should also check the image in the Fast mode window to see if the source is
near an edge. If it is, it's a good idea to examine the light curves from
diffent exposures to verify that they are consistent from exposure to
exposure (while keeping in mind any intrinsic source variability). If the
image is blurred or unusual in any way, users should check the tracking history
file to verify the tracking was reliable.
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